BackgroundThe healthy auditory system has the ability to enable communication in challenging acoustical environments with high levels of background noise. Hearing impairment often leads to a deterioration of this ability, however the degree of which varies widely. It is believed that the specific underlying pathology plays an important part in determining this variability. It has been shown that loss of outer hair cells leads to a reduction in peripheral compression. It has also recently been shown in animal studies that noise over-exposure, producing temporary threshold shifts, can cause auditory nerve fiber (ANF) deafferentation in predominantly low-spontaneous rate (SR) fibers. In this study, auditory steady-state response (ASSR) level growth functions were measured both in normal-hearing (NH) and mildly hearing-impaired (HI) listeners to evaluate the applicability of ASSR to assess peripheral compression and the capacity to code intensity fluctuations at high stimulus levels.

MethodsASSRs were measured using a 64-channel EEG recording amplifier with active electrodes. ASSR level growth functions were measured in adults at stimulus levels between 20 and 90 dB SPL. Subjects were NH, or mildly HI listeners towards higher frequencies. To evaluate compression, sinusoidally amplitude modulated (SAM) tones at carrier frequencies of 0.5, 1.0, 2.0 and 4.0 kHz were presented simultaneously with a modulation depth of 85%. To evaluate intensity coding at high intensities, a single SAM tone with 2 kHz carrier frequency and modulation depths of 25% and 85% was presented.

ResultsFor all listeners at frequencies with normal audiometric thresholds, ASSR level growth functions exhibited consistently compression of about 0.25 dB/dB up to 60 dB SPL. At frequencies with impaired audiometric thresholds, an increased slope of the level growth function, i.e. a reduced compression, was found. For the shallow modulation depth of 25% and levels above 60 dB SPL, the growth function slope showed higher variability across NH subjects than for levels below 60 dB SPL. The same effect was found in HI listeners at the non-impaired frequencies.

ConclusionsThe results show that compression estimates derived from the slope of ASSR level growth functions are similar to other estimates of peripheral compression. Peripheral compression can be estimated simultaneously at four frequencies for levels up to 60 dB SPL, both for NH and HI listeners. Above 60 dB SPL, the results indicate that the slope of the ASSR level growth function obtained using shallow modulation depths provides information about the capacity to code intensity fluctuations.

FundingThis work was funded by the Oticon Center of Excellence for Hearing and Speech Sciences (CHeSS) at the Technical University of Denmark (DTU).

21 Feb 2016

Event (Conference)

Title

Association for Research in Otolaryngology 39th Annual MidWinter Meeting